Cooper W G
International Physics Health & Energy, Inc., Houston, Texas 77030, USA.
Biochem Genet. 1995 Jun;33(5-6):173-81. doi: 10.1007/BF00554729.
A model explaining properties exhibited by fragile-X DNA systems arises from observations that time-dependent base substitutions are expressed at G-C sites but not at A-T sites (Biochem. Genet. 32:383, 1994). [CGG]n sequences are classified as most sensitive to evolutionary base substitution processes involving time-dependent populating of G-C sites with enol-imine states having enhanced stability. Increased density of these states in oocyte DNA would introduce a ground-state collapse double-helix of reduced energy that would inhibit strand separation by the replicase. Evolutionarily altered G' in CG'G triplets allows CG'G to be transcribed as CTG, an initiation codon. And this will cause reinitiation of DNA synthesis, thereby adding additional CGG units to the collapsed double helix. This situation would not occur in slower-evolving male haploid DNA that replicates frequently.
一个解释脆性X DNA系统所表现出特性的模型源于以下观察结果：随时间变化的碱基替换在G-C位点表达，但在A-T位点不表达（《生物化学遗传学》32:383，1994）。[CGG]n序列被归类为对涉及随时间变化用具有增强稳定性的烯醇-亚胺状态填充G-C位点的进化碱基替换过程最为敏感。卵母细胞DNA中这些状态密度的增加会引入能量降低的基态坍缩双螺旋，这将抑制复制酶进行链分离。CG'G三联体中进化改变的G'使得CG'G被转录为CTG，即起始密码子。这将导致DNA合成重新起始，从而向坍缩的双螺旋添加额外的CGG单元。这种情况不会发生在复制频繁的进化较慢的雄性单倍体DNA中。
